1. Field of the Invention
The invention relates to a turbine.
2. Discussion of Background
In the design of axial-flow stages of turbines, two approaches are essentially followed nowadays. Thus, on the one hand, at high work transfer in the stage, a large chord length of the blades and a large hub-section diameter are selected, with at the same time a small blade height. However, this design is inconsistent with the fluid-mechanical knowledge that, in order to reduce leakage losses and wall-friction losses, the blade height selected should be large, with at the same time a small hub diameter, and furthermore that the secondary flow losses increase drastically at a small ratio of blade height to chord length.
In this arrangement of the blades on large hub diameters, the turbine is usually of a chamber type of construction in order to limit the gap losses at the blade tips in the case of the small blade height. As a result, however, the wheel-friction losses greatly increase. Furthermore, the chamber type of construction is very costly. On the other hand, large hub diameters can scarcely be avoided especially in impulse turbines, since otherwise the deflection in the vicinity of the hub increases in such a way that the flow would separate and generate inexcusable losses.
Therefore a further approach selected is to keep the work transfer relatively low and to place large blade lengths on small diameters, the blades being given a small chord length at smaller flow deflection. Due to the substantially smaller hub diameter, the drum type of construction, which is far more cost-effective, may be used. However, a large number of stages result for a machine having given inlet and outlet states of the working medium. This will in turn force the overall length of a machine to be increased, which on the one hand has an adverse effect on the rotor dynamics; on the other hand, the advantage of the lower losses of an individual blade cascade will also be at least partly neutralized again by the large number of stages required. In addition, a type of construction with a large number of stages again pushes the costs up.
For the reasons cited, both design variants are usually combined in steam turbo sets actually constructed. For example, the use of one or more stages of low reaction and with high work transfer at maximum pressures and slightly loaded repeat stages of high reaction in the further course of the expansion of the working medium is widespread. A high pressure in the first stages is rapidly reduced by this type of construction without transmitting significant axial thrust to the rotor, a smaller length of the rotor being necessary for a certain degree of expansion. In this case, for reasons of, in particular, aerodynamic loading, a large chord length of the blades is selected in order not to allow the flow deflection required for achieving the high work transfer to become too extreme. Likewise, the blades are placed on large diameters in order to limit the deflection in the hub section. The further reduction in enthalpy is then effected in stages of high reaction.
Thus the advantages, but also in particular the disadvantages, of both types of construction are combined in modern conventional machines actually constructed. Blading which combines the features of the design variants in such a way that their advantages come to bear without restriction has not been known in technology up to now.